1. Field of the Invention
The present invention relates to a semiconductor device including a thin film transistor over an insulating surface and a manufacturing method thereof.
2. Description of the Related Art
An integrated circuit has been developed, which uses a semiconductor substrate called a silicon-on-insulator (hereinafter also referred to as SOI) which has a thin single-crystal semiconductor layer over an insulating surface, instead of a silicon wafer which is manufactured by thinly slicing an ingot of a single-crystal semiconductor. An integrated circuit using an SOI substrate has attracted attention as a circuit which reduces parasitic capacitance between source and drain regions of the transistor and the substrate, and improves performance of the integrated circuit.
As a method for manufacturing an SOI substrate, a hydrogen ion implantation separation method is known (e.g., see Reference 1: Japanese Published Patent Application No. 2000-124092). A hydrogen ion implantation separation method is a method in which the surface of a silicon wafer is irradiated with hydrogen ions to form a weakened layer at a predetermined depth from the surface of the wafer, a thin silicon layer (a semiconductor layer) which is thinner than the silicon wafer is separated by heat treatment or the like with the weakened layer used as a separation plane (a cleavage plane), and the thin silicon layer (the semiconductor layer) is attached and bonded to another silicon wafer. After an oxide film is formed over the surface of the semiconductor layer by heat treatment in an oxidizing atmosphere, it is necessary to remove the oxide film as well as the weakened layer remaining on the separation plane and then to perform heat treatment at 1000 to 1300° C. in a reducing atmosphere to increase bonding strength at an attachment interface.
Further, a semiconductor device in which a single-crystal semiconductor layer is formed over an insulating substrate such as a high heat-resistant glass substrate is disclosed (e.g., see Reference 2: Japanese Published Patent Application No. H11-163363) The semiconductor device has a structure in which the entire surface of the glass substrate is protected by an insulating silicon film and the thin single-crystal semiconductor layer obtained by the hydrogen ion implantation separation method is firmly fixed over the insulating silicon film.
Meanwhile when strain is applied to a semiconductor layer, mobility can be considerably improved when a transistor is manufactured with the semiconductor layer used as an active layer compared to the case where a transistor is manufactured with a normal semiconductor layer used as an active layer (e.g., see Reference 3: IEDM Tech. Digest, 1994, pp. 373 to 376 and Reference 4: IEEE ELECTRON DEVICE LETTERS, VOL. 15, No. 10, 1994, pp. 402 to 405). For example, in an N-channel transistor, when a semiconductor layer to which tensile strain is applied is used as an active layer, electron mobility can be considerably improved. On the other hand, in a P-channel transistor, when a semiconductor layer to which compressive strain is applied is used as an active layer, hole mobility can be considerably improved.